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#define NVIC_ST_CTRL_R      (*((volatile unsigned long *)0xE000E010))
#define NVIC_ST_RELOAD_R    (*((volatile unsigned long *)0xE000E014))
#define NVIC_ST_CURRENT_R   (*((volatile unsigned long *)0xE000E018))
void SysTick_Init(void){
  NVIC_ST_CTRL_R = 0;              // 1) disable SysTick during setup
  NVIC_ST_RELOAD_R = 0x00FFFFFF;   // 2) maximum reload value
  NVIC_ST_CURRENT_R = 0;           // 3) any write to current clears it
  NVIC_ST_CTRL_R = 0x00000005;     // 4) enable SysTick with core clock
}

unsigned long Now;      // 24-bit time at this call (12.5ns)
unsigned long Last;     // 24-bit time at previous call (12.5ns)
unsigned long Elapsed;  // 24-bit time between calls (12.5ns)
void Action(void){      // function under test
  Now = NVIC_ST_CURRENT_R;         // what time is it now?
  Elapsed = (Last-Now)&0x00FFFFFF; // 24-bit difference
  Last = Now;                      // set up for next...
}

#define NVIC_ST_CTRL_R      (*((volatile unsigned long *)0xE000E010))
#define NVIC_ST_RELOAD_R    (*((volatile unsigned long *)0xE000E014))
#define NVIC_ST_CURRENT_R   (*((volatile unsigned long *)0xE000E018))
void SysTick_Init(void){
  NVIC_ST_CTRL_R = 0;               // disable SysTick during setup
  NVIC_ST_CTRL_R = 0x00000005;      // enable SysTick with core clock
}

// The delay parameter is in units of the 80 MHz core clock. (12.5 ns)
void SysTick_Wait(unsigned long delay){
  NVIC_ST_RELOAD_R = delay-1;  // number of counts to wait
  NVIC_ST_CURRENT_R = 0;       // any value written to CURRENT clears
  while((NVIC_ST_CTRL_R&0x00010000)==0){ // wait for count flag
  }
}

// 800000*12.5ns equals 10ms
void SysTick_Wait10ms(unsigned long delay){
  unsigned long i;
  for(i=0; i<delay; i++){
    SysTick_Wait(800000);  // wait 10ms
  }
}